The present invention relates to control of ground plane voltages in electronic systems. In particular, the present invention relates to control of ground plane voltages over extensive ground plane areas through the use of voltage sensors and drivers.
In electronic systems, a ground establishes a reference that is the basis for determining the magnitude of voltages present at other points in the circuitry. However, typically there is not a single ground point. Rather, the electronic system may be built, for example, using a multi-layer printed circuit board in which one or more layers is a solid copper ground layer. Individual electronic components are distributed on the top and bottom (outermost layers) of the circuit board and connected to the ground plane through via(s) that terminate on the ground plane(s). During operation of the electronic system, currents flow through the electronic components and into or out of the ground plane.
The solid copper ground plane (more generally, any distribution of ground traces) has a small but finite resistance between any two points. As a result, ground plane currents will create potential differences in the ground plane itself according to the net vector ground plane current. Therefore, a voltage measurement elsewhere in the electronic system depends on the particular point in the ground plane used as the reference. The difference in measured voltage can be quite significant between two distinct points on the ground plane, often on the order of several millivolts.
While certain systems may be relatively immune to a change in voltage of a few millivolts, other, more sensitive systems can be dramatically effected. For example, the extreme sensitivity of Charge Coupled Devices and solid state X-ray detectors renders them very susceptible to variation in ground plane potential. Very small spatial differences in ground plane potential can cause image artifacts during the readout of the detector. Generally, the variation in ground plane potential includes a DC and an AC component.
The DC component of the variation in ground plane potential may sometimes be zeroed out by subtracting a reference (or xe2x80x9cdarkxe2x80x9d) image from a subsequently captured image. This reference (dark) image is made at a time when there is no x-ray illumination, and therefore contains only static (DC) offset information. Naturally, additional processing complexity and processing time are required to perform dark image subtraction. Furthermore, the AC component of the variation in ground plane potential is not corrected by dark image subtraction. As a result, even complex electronic systems that perform dark image subtraction remain susceptible to image artifacts which are caused by changing (AC) currents and voltages. As examples, the image artifacts in x-ray images can result in reduced image quality, reduced diagnostic usefulness, and inconsistent imaging of the same target.
A need has long existed for a method and apparatus that addresses the problems noted above and others previously experienced.
A preferred embodiment of the present invention provides a ground voltage control system. The ground plane voltage control system includes a working ground reference, and an isolated ground reference connected to the working ground reference. The isolated ground reference is connected such that current flow in the isolated ground reference is reduced below a predetermined threshold in order to keep the voltage gradients at an acceptably low level. The ground plane voltage control system also includes a voltage controller. The voltage controller includes a first sense input connected to the isolated ground reference, a second sense input connected to the working ground reference, and a controller output connected to the working ground reference. The controller output carries a voltage compensation signal to drive a current into or out of the working ground reference to offset a voltage difference sensed between the isolated ground reference and the working ground reference.
Another preferred embodiment of the present invention provides a method for controlling ground plane voltage. The method includes sensing a voltage difference between a working ground reference and an isolated ground reference, generating a voltage compensation signal based on the voltage difference, and driving the working ground reference with the voltage compensation current signal in order to reduce the voltage difference between the isolated ground reference and the working ground reference at the point where the voltage of the working ground reference was sampled.